1. Field of the Invention
The invention relates to an electric device and/or a magnetic device, and more particularly to the design of a Wien Filter or called as ExB for all the applications, such as deflecting charged particle beam in electron microscope or separating charged particles in dependence upon mass, or energy, or moving direction thereof.
2. Description of the Prior Art
Wien filter is known for its capability to separate charged particles in dependence upon mass, or energy, or moving direction thereof. It is based on the principle that the magnetic force of a magnetic field acting on a charged particle depends on the velocity vector thereof, but the electric force of an electric field acting on a charged particle does not. If the vector directions of the electric field E, magnetic field B and the particle velocity ν are perpendicular to each other and obey the right-hand rule, the electric force and magnetic force acting on the particle will be opposite to each other and perpendicular to the moving direction of the particle, thereby generating a total force as shown in Equation (1.1).F=q·(E−ν·B)  (1.1)
Furthermore, if the magnitudes of the foregoing three vectors meet a specific condition, which is called as Wien Condition and shown in Equation (1.2), the net force exerted on the particle will be zero, thereby not changing the moving of the particle. Any other particle, different from said particle in velocity vector, will receive a non-zero net force, and then will be deflected away from its original moving direction.E−θ·B=0  (1.2)
A primary configuration of Wien filter is shown in FIG. 1, where the electron is taken as an example of charged particles. A magnetic dipole field B1 and an electric dipole field E1 are respectively generated by a pair of magnetic pole-pieces 1 and 2 with opposite polarities and a pair of electrodes 3 and 4 with opposite potentials. Both of the dipole fields are perpendicular to each other, and superimposed along and perpendicular to a straight optical axis which lies on Z-axis. Each magnetic pole-piece may be formed by a permanent magnet or electromagnetic magnet. For sake of simplicity, hereinafter the reference regarding a magnet is expressed by the related magnetic pole-piece thereof. In the Wien filter shown in FIG. 1, two grounded pole-pieces, which include a N pole-piece 1 and a S pole-piece 2, will generate a magnetic dipole field B1 in Y direction, while two electrodes, which include an electrode 3 at +Vd potential and an electrode 4 at −Vd potential, will generate an electric dipole field E1 in X direction. For the electrons moving along the Z-axis and with a same velocity ν0, the magnetic and electric dipole fields B1 and E1, which respectively exert the electric force Fe and magnetic force Fm on these particles, are set to meet Wien condition as shown in Equation (1.2).
For a particle, its velocity ν is related to its mass m and kinetic energy V, as shown in Equation (1.3). Obviously, Wien Condition can be satisfied for particles having a given ratio between mass and energy and a given moving direction. Therefore a Wien filter can be operated as a mass separator for particles identical in charge and energy but with a mass range, or an energy separator for particles identical in charge and mass but with an energy spread. Besides, a Wien filter can also act as a beam separator for two charged particle beams both having particles identical in mass and charge but traveling in mutually opposite directions such as primary beam and secondary beam in SEM (for example U.S. Pat. No. 4,658,136). Furthermore, a mass separator can be used for mass analysis or as a mass filter, and analogously an energy separator for energy analysis of material or as an energy filter or called as monochromator (for example U.S. Pat. No. 5,838,004).
                    v        =                                            2              ·              V                        m                                              (        1.3        )            
For most of the applications of Wien filter, Wien filter is employed in an imaging system, wherein the straight optical axis of the Wien filter coincides with the straight optical axis of the imaging system. If the Wien condition is not satisfied wherever the particles of the imaging beam will go through, the additional aberrations will be added to the imaging beam due to the undesired particle deflection. Therefore, constraining or even eliminating the adverse impact of Wien filter on imaging quality is a prerequisite for employing a Wien filter in such a case, and meeting Wien Condition to the maximum extent possible is the essential requirement for constructing a Wien Filter in practical use. Meeting Wien condition in practice can be considered separately in the on-axis (on optical axis) and the off-axis (off optical axis) areas.
At first, in the on-axis area, the velocities of on-axis particles are constant because there is no axial acceleration or deceleration field within Wien filter. Therefore Equation (1.2) requires the on-axis electric and magnetic fields E1 and B1 have a same distribution shape. If it is not true, the net forces exerted on the on-axis charged particles will not be zero, thereby gradually deflecting the particles away from the optical axis and generating off-axis aberrations. The better the two fields match each other in field distribution shape, the smaller the net forces will be, thereby appearing the less off-axis aberrations. However, fundamentally the electric and magnetic fields impossibly match each other perfectly if the electrodes in the electric deflector and the magnetic pole-pieces in the magnetic deflector are not identical in geometry.
Secondly, in the off-axis area, due to a potential change in the electric field direction which is X direction in FIG. 1, any off-axis particle in the imaging beam will have a velocity not only different from the given velocity of the on-axis particles but also dependent on its off-axis shift in X direction, as shown in Equation (1.4). Fundamentally, if both of the electric and magnetic dipole fields are uniform in the electric field direction, the Wien condition can not be satisfied over the entire off-axis area, thereby leading to a focusing effect in this direction and hence adding astigmatism to the imaging particle beam.
                                          v            0                    -          v                =                  -                                    e              ·                              E                1                            ·              x                                      m              ·                              v                0                                                                        (        1.4        )            
A number of methods have been proposed to construct a Wien filter meeting the Wien Condition to the maximum extent possible. One way is to make the magnetic dipole field have a magnitude gradient in the electric field direction by using a pair of magnetic pole-pieces with wedge front end or hyperbolic front end such as U.S. Pat. No. 4,924,090 and U.S. Pat. No. 5,444,243, or adding a coil to the flat front end of each pole-piece such as U.S. Pat. No. 4,019,989. Another way is to use additional quadrupole field to compensate the astigmatism by using a multi-pole type Wien filter which can form a quadrupole field as well as a dipole field. The later has achieved wide acceptance because of its flexibility of matching the changeable operation conditions of the imaging systems which Wien filter will be applied to. Usually, it is realized by using an electric or/and a magnetic multi-pole device in a Wien filter, so as to act as a deflector and a stigmator simultaneously. However, when a multi-pole device is excited to generate a dipole field, it actually generates a field which not only comprises a dipole field or called as 1st order harmonic but also many higher order harmonics which are undesired due to incurring aberrations. The first higher order harmonic is 3rd order harmonic.
Tian-Tong Tang (Optik, 74, No. 2, 1986, P51-56) proposes an 8-pole type Wien filter in which each of eight identical magnetic pole-pieces is also used as an electrode. This arrangement fundamentally ensures a good match of magnetic and electric dipole fields in field distribution shape, and its excitation way minimizes the 3rd order harmonics of electric and magnetic fields. Similarly, Lopez and Tsuno (U.S. Pat. No. 6,844,548) provide a Wien filter having twelve identical pole-pieces as both of magnetic pole-pieces and electrodes, which minimizes the undesired the 3rd field harmonics by an excitation way requiring fewer power supplies than the former.
On the entrance and exit sides of a Wien filter, the distributions of the magnetic and electric dipole fields more depend on the corresponding measures for field termination. As shown in FIG. 2(a), a usual measure is putting two grounded terminating plates 7 and 8 to sandwich the electrodes (3 and 4) and magnetic pole-pieces (1 and 2) with two axial gaps 15 and 16. The terminating plates are made of a material of both electric and magnetic conductor and hence can effectively constrain the electric and magnetic fields to be between them, i.e. within the main area 10 and two near fringe areas 11 and 12. However, due to the opening on each terminating plate for particles passing through, the electric field and magnetic field can not be terminated perfectly, and will leak out to the far fringe areas 13 and 14. In addition, the electric field will weaken faster than the magnetic field. Therefore, the distribution shapes of the electric and magnetic field will be almost same in the main area 10, but have a larger difference in the near fringe areas 11 and 12 and a smaller difference in the far fringe areas 13 and 14, as shown in FIG. 2(b). Usually, the lengths of the electrodes and magnetic pole-pieces are designed to be larger than the inner diameters thereof, so that the range with a good fields-match will dominate the entire field range.
In practice, a compact design of Wien filter is also important for many applications. For example, a longer Wien filter is not useful for an energy filter used to improve imaging resolution in SEM because of incurring a larger electron interaction; a Wien filter having larger outer radial dimensions is unaccepted for a beam separator employed to improve collection of secondary electron in SEM because its desired location is inside the bore of the magnetic objective lens. A Wien filter having a shorter length but a good fields-match is proposed in U.S. Pat. No. 6,452,169, which requires the mutual distance of the electrodes and the mutual distance of the magnetic poles are larger or approximately equal to the mutual distance of the terminating plates. However its requirement limits the efficiency of the Wien filter. A Wien filter having a compact structure is used in U.S. Pat. No. 4,658,136, which comprises an electric deflector having four cylindrical identical electrodes 3, 4, 5, and 6 and a magnetic deflector having two saddle coils 1 and 2 covering the outer sidewall of the four electrodes as shown in FIG. 3, wherein the electrodes 5 and 6 are grounded and the electrodes 3 and 4 are at +Vd and −Vd potential respectively. An electric dipole field E1 in X direction is then generated by the four electrodes 3, 4, 5, and 6. The two coils 1 and 2 will generate a magnetic dipole field B1 in Y direction. For this structure, it is difficult to obtain a good fields-match because the magnetic field distribution depends on the coil shape which is difficult to be made accurately.
Accordingly, a new design of Wien filter, which can meet the Wien condition as much as possible and has a compact and efficient structure, is demanded by many applications, particularly for a charged particle apparatus using Wien filter in its imaging system such as a SEM.